[单选题]
The world is not only hungry; it is also thirsty for water. This may seem strange to you, since nearly 75% of the earth’s surface is covered with water. But about 97% of this huge amount is sea-water, or salt water. Man can only drink and use the other 3%---the fresh water that comes from rivers, lakes, underground, and other sources. And we cannot even use all of that, because some of it is in the form of icebergs（冰山）and glaciers（冰川）. Even worse, some of it has been polluted.What percentage of the earth’s water can man actually use at present?
A .Nearly 75 per cent.
B .About 97 per cent.
C .Exactly 3 per cent.
D .Less than 3 per cent.
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